He made the comparison as he appealed for Ukip supporters to vote Tory on Thursday.

Johnson told The Sun that a Labour-SNP government would be a "supine, useless, federalist" one and went one further in typical BoJ style to say: "Sturgeon will be the poodle of Brussels and Miliband will be the chihuahua. If you want change in Europe you have got to vote Conservative.”

The London Mayor, who is standing to be an MP again, also told The Sun his party would give people a say on Europe and immigration - two big policy pledges by Nigel Farage's Ukip.

He said: "My message to Kippers is this: grit your teeth, screw up your eyes, take a deep breath and do the right thing for this country.

“If I have anything to do with it, as a representative of the people of Uxbridge and South Ruislip as I hope to be, I will bloody well make sure we have a referendum this time.”

He predicted Ukip would win "very few seats", but added they would still take a lot of votes from the Tories: "On current projections, they will still take a huge number of votes from the party that could deliver what they want — us.”